The present invention is directed to detection of a component gas contained in a mixture of gases. More specifically, the invention is a method, and apparatus that implements that method, that detects helium tracer gas for locating leaks in underground telephone cables.
Telephone cables, to prevent water from shorting pairs of the wires that make up the cables, are usually enclosed in a sheath and internally pressurized with dry air compressors placed at predetermined locations along the cable. Unfortunately, there are times when the sheath is penetrated, either by chemical degradation (electrolysis), mechanical stresses, or similar phenomena, causing leaks in the sheath and allowing the introduction of water. This, in turn, causes a degradation of any internal insulation, permitting deterioration and shorting of the internal wiring, and degradation of the signals carried by the cable. Repairing this damage can cost substantial sums because it is sometimes necessary to excavate in several points or trench hundreds of yards of cables due to the impossibility of identifying the exact location of the leak.
Recent techniques have been developed to more accurately pinpoint the location of leaks that develop in the cable sheath in order to minimize the amount of cable that must be unearthed and replaced. One technique that has been found sufficiently successful to enjoy common use today is one that includes the step of obtaining a preliminary rough location of the leak by taking pressure measurements of the cable. The resolution of this technique can locate the leak within anywhere from 300 feet to 6,000 feet. The cable, at periodic locations, is then pressurized with a helium tracer gas, and a helium detector used to pinpoint the leak more accurately by detecting the helium that permeates to the surface of the ground from the leak. One technique of detection is by use of a helium mass spectrometer.
While the use of helium mass spectrometers have been somewhat reliable, mass spectrometers themselves are not without certain problems. Their operation depends upon separation of the helium gas in a vacuum by imparting an electrical charge to the gas sample containing the helium, pushing the sample through a magnetic field, and collecting the hydrogen ions from the result. The electronics, including a supply of high voltage, and the vacuum system make this type of helium detector bulky, complicated and expensive. Since leaks are just as prone to developing in relatively remote areas where access to such heavy equipment can be difficult, the efficacy of this leak-detection technique becomes limited.
Helium tracer gas detection is also performed using negative-going combustible gas detectors. These devices, however, lack the sensitivity necessary for use in tracer gas detection, making this type of detection technique less effective than desired.